1. Technical Field
The present invention relates to a pulse wave detector, and more specifically, to a pulse wave detector for detecting pulse waves by sending and receiving ultrasonic waves to and from an artery.
2. Related Art
Detection of pulse waves caused by blood flow through an artery is widely used in cases such as medical institutions and healthcare administration. This pulse wave detection is also widely used for automatic electronic detection of pulse rate etc., as well as for detecting pulse rate at a specified time by touching an artery with a finger.
As a device for obtaining pulse rate by electronically detecting pulse waves, devices currently exist that have a piezoelectric element arranged on an artery and pulse rate is detected from pressure variations on the skin (positional variations of the skin caused by pressure) accompanying pressure variations inside the artery, or which use ultrasonic waves to detect pulse rate.
Japanese patent laid-open No. Hei. 1-214335 and U.S. Pat. No. 4,086,916, for example, propose use of the Doppler effect caused by blood flow, as a pulse wave detector that uses ultrasonic waves.
FIG. 10 shows the appearance of frequency variations of ultrasonic waves due to this Doppler effect.
If an ultrasonic wave as shown in FIG. 10A having frequency f0 is transmitted from a body surface towards an artery, the transmitted ultrasonic wave is reflected by blood flowing in the artery. If this reflected wave is received by a receiver it is possible to detect variations in the frequency of the reflected wave. Specifically, if the frequency of a received wave is made f1, since the blood flow velocity in the artery is fast in the systolic phase of the heart, the frequency of the reflected wave becomes higher due to the Doppler effect (A part), while conversely the frequency becomes lower than the A part during relaxation of the heart (B part) since the blood flow velocity is lower, as shown in FIG. 10B.
In this way, ultrasonic waves are directed to blood flow in an artery that changes in flow velocity due to beating of the heart, and pulse waves are detected by detecting frequency variation, and it is also possible to detect pulse rate and blood flow velocity.
Also, in German Patent publication No. 3345739 there is proposed a pulse wave detector that uses a plurality of sensors (a plurality of groups of sensors), and FIG. 11 shows the arrangement of a sensor in this type of pulse rate detector. A transmitter 11a and a receiver 21a inside a sensor 19a are rectangular in shape. The longer side of each rectangle is arranged so that it is parallel to blood flow in an artery 2, and so that a line connecting the transmitter 11a and the receiver 21a is orthogonal to the artery 2.
However, with pulse wave detectors for detecting pulse waves using ultrasonic wave Doppler effects, as in the inventions disclosed in Japanese Patent laid-open No. Hei. 1-214335 and U.S. Pat. No. 4,086,916, there is a problem that power consumption is extremely large because ultrasonic waves are used.
As a result, the pulse wave detectors of the related art must be used in an environment such as a hospital or a house where electric power can be adequately supplied, and there is a problem that when they are used in any other environment ultrasonic waves can only be measured for a short time.
Particularly, in the case of a pulse wave detector that is of such a size and weight as to be portable, for instance a pulse wave detector built into a wristwatch, since the battery capacity is limited there is a problem that usage time is made even shorter.
Also, the ultrasonic waves f0 transmitted for pulse wave detection are progressive waves in the order of a few MHz. This means that with a pulse wave detector, such as the invention disclosed in German patent publication No. 3345739, having a long side of a transmitter 11a and receiver 21a arranged parallel to the blood flow of an artery 2, it is necessary to accurately transmit the ultrasonic waves f0 toward the artery 2, and it is difficult to align the position of the sensor 19a. Also, even if the positional alignment is performed accurately, there is a problem that it becomes impossible to measure ultrasonic waves due to positional variations in the artery 2 and the sensor 19 with movement of the wrist 2a. 
The present invention has been conceived in order to solve the above described problems, and a first object of the invention is to provide a pulse wave detector that can detect pulse waves with low power consumption, and which is capable of prolonging usage time.
A second object of the present invention is to provide a pulse wave detector that enables simple alignment of a sensor with an artery, and is capable of carrying out pulse wave detection even when there is wrist movement.
A pulse wave detector of the present invention comprises a transmitter for transmitting ultrasonic waves toward an artery, a receiver for receiving ultrasonic waves transmitted from the transmitter and reflected by blood flowing in the artery, a drive controller for intermittently driving at least one of the transmitter and the receiver, a pulse wave information acquisition unit for acquiring pulse wave information relating to pulse waves from the ultrasonic waves received by the receiver, and an output unit for outputting the pulse wave information acquired by the pulse wave information acquisition unit.
By intermittently driving at least one of the transmitter or the receiver in this way, it is possible to suppress the power consumption to the drive duty cycle. Specifically, a pulse wave detector has low power consumption. For example, by building the pulse wave detector into a watch, it is possible to routinely prolong usage. In this case, it is possible to utilize part or all of an oscillator section used in the watch as the drive controller of the present invention, and in this way an even simpler construction is made possible.
With the pulse wave detector of the present invention, it is possible for the pulse wave information acquisition unit to be provided with a memory for storing pulse wave information, and for the output unit to output the pulse wave information stored in the memory. Specifically, pulse rate information and detection information for specified time sections are stored in the memory in advance, and can be used for medical diagnosis by outputting to an external device such as a medical diagnostic machine.
With the pulse wave detector of the present invention, it is also possible for the pulse wave acquisition unit to acquire a pulse rate from the detection signal as pulse wave information, and for the output unit to output the pulse rate acquired by the pulse wave information acquisition unit. In this way, it is possible to routinely confirm a pulse under normal conditions.
The pulse wave detector of the present invention may also be provided with a display, and it is possible for the pulse wave information acquisition unit to acquire a pulse rate or pulse wave waveform as information relating to a pulse wave from the detection signal, and for the output unit to output the pulse rate or pulse wave waveform acquired by the pulse wave information unit to the display. In this way, by displaying the pulse rate or the pulse wave waveform, it is possible to easily confirm the pulse rate or pulse wave form even during normal day to day activities.
According to another aspect of a pulse wave detector of the present invention, the pulse wave information acquisition unit has a frequency detector for detecting frequency variations of the ultrasonic waves received by the receiver, and pulse wave information is acquired from a detection signal from the frequency detector.
In the pulse wave detector of the present invention, the drive controller intermittently drives both the transmitter and the receiver, and varies the drive timing of the transmitter and the drive means of the receiver. By intermittently driving both the transmitter and the receiver in this way, it is possible to lower power consumption. Also, by making the drive timing of the transmitter and the receiver adjustable, it is possible to adjust the rise times of the transmitter and the receiver to optimum conditions. For example, by starting drive of the receiver a specified time after drive of the transmitter, it is possible to prevent the receiver receiving ultrasonic waves during a time period from start-up of the transmitter until output of ultrasonic waves is stable.
In the pulse wave detector of the present invention, the drive controller intermittently drives both the transmitter and the receiver, and varies the drive times of the transmitter and the drive means of the receiver. By making the time for which the transmitter and the receiver are driven independently adjustable, it is possible, for example, to reduce the drive time of the receiver and reliable receive stable ultrasonic waves. It is also possible to reliably receive all transmitted ultrasonic waves by prolonging the time for which the receiver is driven.
Also, in the pulse wave detector of the present invention, the drive controller varies the drive time and off time for intermittent drive. By making the drive time and the off time adjustable, it is possible to achieve optimum drive while reducing power consumption.
In the pulse wave detector of the present invention, the drive controller intermittently drive at a frequency at least double a maximum assumed heart rate. For example, if the assumed maximum heart rate is 240 beats per minute, at least one of the transmitter and the receiver are intermittently driven at a frequency of 8 Hz or more. Since intermittent drive is always performed at a frequency at least double the frequency to be detected in this way, it is always possible to detect pulse waves in a stable manner. In this case, even if the assumed heart rate is low (an upper limit of 100 beats per minute when at rest) intermittent drive is performed at the same frequency of 8 Hz.
In the pulse wave detector of the present invention, the drive controller performs intermittent drive at a frequency at least double the frequency of the commercial power supply. Specifically, by performing intermittent drive at a frequency of 120 Hz which is at least double the 50 Hz or 60 Hz of a commercial power supply, it is possible to protect against the effects of noise due to the commercial frequency. In this case, by setting the intermittent drive frequency to 128 Hz, it is possible to-possible to divide the oscillating frequency 32 kHz of an oscillator used in a watch to provide the intermittent drive frequency of 128 Hz, making a simple construction possible when the pulse wave detector is implemented in a watch.
In the pulse wave detector of the present invention, the drive controller also performs intermittent drive at a frequency at least double the frequency of a commercial power supply, and at a frequency having an extremely low duty ratio.
A further aspect of the pulse wave detector of the present invention comprises a transmitter for transmitting ultrasonic waves toward an artery, a receiver for receiving ultrasonic waves transmitted from the transmitter and reflected by blood flowing in the artery, a pulse wave information acquisition unit for acquiring pulse wave information relating to pulse waves from the ultrasonic waves received by the receiver, and an output unit for outputting the pulse wave information acquired by the pulse wave information acquisition unit. An ultrasonic wave transmitting surface of the transmitter and an ultrasonic wave receiving surface of the receiver are formed long and narrow in shape having a long axis and a short axis, with the long axis being arranged so as to cross the artery.
A still further aspect of the pulse wave detector of the present invention comprises a transmitter for transmitting ultrasonic waves toward an artery, a receiver for receiving ultrasonic waves transmitted from the transmitter and reflected by blood flowing in the artery, a drive controller for intermittently driving at least one of the transmitter and the receiver, a pulse wave information acquisition unit for acquiring pulse wave information relating to pulse waves from the ultrasonic waves received by the receiver, and an output unit for outputting the pulse wave information acquired by the pulse wave information acquisition unit. An ultrasonic wave transmitting surface of the transmitter and an ultrasonic wave receiving surface of the receiver are formed long and narrow in shape having a long axis and a short axis, with the long axis being arranged so as to cross the artery.
By making the transmitting surface of the transmitter and the receiving surface of the receiver long and narrow in shape in this way , with a long axis and a short axis, the transmitter and the receiver are positioned over an artery even if the artery slides in a lateral range of the circumference of the wrist, which means that there is no need to correct the position of the transmitter and the receiver, and it is possible to continue measuring pulse waves.
Also, since the transmitter and the receiver are arranged so that the long axis intersects the arterial blood flow direction, it is possible to transmit ultrasonic waves towards the artery and to receive reflected waves without specially correcting a sensor position, even if the sensor deviates from the artery along the circumference of the wrist, as long as the amount of deviation is within the range of the long axis.
Further, it is possible to affix a strap to a user""s wrist without a user paying particularly attention to placement of the transmitter and receiver over the artery.